Steam generator

ABSTRACT

A steam generator includes a U-shaped nest of tubes having vertical legs with their lower ends mounted in the holes of a circular tube sheet and centrally forming a vertical corridor or passage extending upwardly from the tube sheet, the nest and the upper surface of the tube sheet being enclosed for containment of water and generated steam. Feed water flows radially inwardly from the periphery of the tube sheet towards its center, the heating fluid flowing through the tube&#39;&#39;s nest via the tube sheet&#39;&#39;s lower surface. To prevent the feed water from prematurely converting to steam, or boiling, on the upper surface of the tube sheet and in the corridor or passage, means are provided for reinforcing the inward radial flow force of the feed water, assuring an adequate flow of the feed water throughout those areas to prevent such premature boiling or conversion to steam.

'United States Patent [191 [111 3,907,029

Klein et al. Sept. 23, 1975 STEAM GENERATOR Primary ExaminerCharles J.Myhre [75] Inventors: Heinrich Klein, Erlangen; Eduard AssistantExaminer Theophfl Streule Weber, Numberg, both of Germany Attorney,Agent, or Firml(enyon & Kenyon Reilly Carr & Chapm [73] Assignee:Siemens Aktiengesellschaft, Munich,

Germany [57] ABSTRACT [22] Filed: Aug. 29, 1973 i A steam generatorincludes a U-shaped nest of tubes PP N05 392,523 having vertical legswith their lower ends mounted in the holes of a circular tube sheet andcentrally form- [30] Foreign Application Priority Data ing a verticalcorridor or passage extending upwardly S t 4 1972 G 2243417 from thetube sheet, the nest and the upper surface of 6p ermany the tube sheetbeing enclosed for containment of water and generated steam. Feed waterflows radially l65/l6ll,28lf2;g; inwardly from the periphery of the tubesheet towards Fie'ld 159 16]. its center, the heating fluid flowingthrough the tubes nest via the tube sheets lower surface. To prevent thefeed water from prematurely converting to steam, or boiling, on theupper surface of the tube sheet and in the corridor or passage, meansare provided for reinforcing the inward radial flow force of the feedwater,

[56] References Cited UNITED STATES PATENTS 3,238,729 3/1966 Frankel etal 165/161 assuring an adequate flow of the feed water through-3,387,652 6/1968 Drobka 165/162 out those ar as to prevent suchpremature boiling or 3,503,373 3/1970 Sukolowski 122/32 conversion tosteam 3,566,961 3/1971 Lorenz 165/159 1 Claim, 3 Drawing Figures USPatent Sept. 23,1975 Sheet 1 of 2 3,907,029

STEAM GENERATOR BACKGROUND OF THE INVENTION A nuclear reactor usingpressurized-water coolant produces useful power via a steam generatorcharacteristically comprising an upstanding, inverted, U-shaped nest oftubes through which the coolant is passed as a heating fluid, the tubeshaving vertical legs with lower ends mounted in the holes of ahorizontal circular tube sheet and centrally forming a vertical corridoror passage. An enclosure for the tube sheets upper surface and the tubenest contains water and generated steam. The enclosure has a steamoutlet in its top and means for flowing feed water into the enclosureradially inwardly from the periphery of the tube sheet and over thelatters upper surface for conversion to steam by upward circulationthrough the tube nest. Two chambers below the tube sheet arerespectively provided with a coolant inlet and outlet, the two chambersconnecting respectively with inlet and outlet ends of the tube neststubes. Therefore, the coolant flow is via the SUMMARY OF THE INVENTIONOne object of the invention is to prevent such premature conversion ofthe water to steam, or localized boiling, in a steam generator of thetype described.

According to the invention, this object is attained by providing such asteam generator with means for reinforcing the inward radial flow forceof the feed water over the upper surface of the tube sheet and withinthe lower portion of the passage or corridor formed by the tube nest, toan extent preventing the waters conversion to steam, or boiling, locallyadjacent to the center portion of the flow over the tube sheet andupwardly through the passage or corridor. By reinforcing this radialinward flow component, the volume of the feed water flowing over thecentral portion of the tube sheet and upwardly is increased relative tothat provided by prior art constructions of this type of steamgenerator.

In one form the reinforcing means comprise making the tube pitch of thevertical legs of the tubes of the tube nest, smaller around theperiphery of the tube sheet than at its central portion. In other words,the tubes vertical legs are interspaced more closely together around theperiphery of the tube sheet than at its central portion. Thisconstruction inherently provides flow resistance to the upward flow ofthe feed water around the peripheral portion of the tube sheet, thusincreasing the inward flow force or component of the feed water,resulting in an increase in the volume of feed water that reaches thecenter of the tube sheet and flows upwardly through the tube nestcenter.

When introduced to the steam generator the feed water, of course, has arelatively low temperature, substantially below its boiling temperatureat the pressures involved, and with the present invention, the feedwater remains in its liquid phase throughout not only the peripheralportion of the tube sheet and tube nest, but centrally as well, as thefeed water flows upwardly from the tube sheet through the tube nest,thus eliminating, or at least substantially reducing, the risk that thefeed water will locally prematurely convert to steam so as to result inthe buildup of undesirable deposits.

In another form, the feed water flow reinforcing means comprise a seriesof vertically interspaced annular plates having outer peripheralproportions of substantially the same diameter positioned to receive theinitial feed water flow, and having inner diameters which progressivelyincrease from the uppermost one of this series of plates downwardly tothe lowermost one. With this arrangement the radial inward flow of feedwater is held against upward flow around the peripheral portion of thetube sheet and tube nests while inwardly being gradually released forupward flow.

In still another form, the flow reinforcing means comprise the uppersurface of the tube sheet having substantially a convex surface. Thismay be done by making the entire tube sheet convex and withsubstantially the same thickness throughout, the bottom of the tubesheet then having a corresponding convexity, as by thickening the tubesheet appropriately, in which case the bottom of the tube sheet is aplane or flat surface.

BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention areschematically illustrated by the accompanying drawings in which:

FIG. 1 in vertical section shows the essential portions of a steamgenerator to the extent required to understand the invention;

FIG. 2 is like FIG. 1 but shows a second example; and

FIG. 3 illustrates a third example.

DETAILED DESCRIPTION 'OF THE INVENTION Having reference to the abovedrawings, FIG. 1 shows a steam generator 1 of the type used forgeneration of steam from the heat of a pressurized-water coolant incircuit with the pressure vessel of a nuclear reactor using such acoolant. I

Thus, FIG. 1 shows the inverted U-shaped nest of tubes having thevertical inlet legs 3 and corresponding outlet legs 4, the innermost ofthe legs 3 and 4 necessarily forming a vertical passage or corridor 5.The lower ends of the legs 3 and 4 are mounted in the holes of acircular, horizontal tube sheet 8 which divides the lower end of thecylindrical wall 9 of the generators casing. Although not shown, in theusual way this casing has its upper end formed as a steam dome having anoutlet for the steam. Spaced inwardly from the wall 9, a cylindricalshell 10 separates the boiling space 11 in which the feed water rises inthe direction shown by the arrow 11a. The annular space 12 between thecylindrical walls of the two parts 9 and 10 form the descent space 12down through which feed water descends for preheating and thermalcirculation of the water; a feed water inlet (not shown) located in theupper part of the steam generator casing or housing maintains an inputof water.

The lower end 13 of the shell 10 is spaced above an outer peripheralportion of the tube sheet 8, the feed water descending through the space12 and, as indicated by the arrow 14, flowing radially over the uppersurface of the tube sheet 15 for upward flow as indicated by the arrow 11a. This radial flow of water should reach the center 15 of the tubesheet 8 and flow upwardly without initially boiling, until it reaches ahigher portion of the boiling space 11, if the undesirable dcposits areto be avoided.

Beneath the tube sheet 8 the steam generators casing or housing forms ahemispherical chamber 9a divided by a wall 9b into inlet and outletcoolant manifolds 9c and 9d respectively. Therefore, the coolant flowthrough the tubes of the tube nest is via the lower surface of the tubesheet 8, resulting in the upper surface of the tube sheet havingrelatively higher temperatures. In addition, the innermost ones of thevertical tube legs 3 and 4 of the tube nest, provide shorter coolantflow paths than do the outermost ones of these legs. If the radialinward feed water flow indicated by the arrow 14 is inadequate toprovide a sufficient flow to the center 15 of the tube sheet 8 andupwardly through the tube sheet, the water will convert to steam or boilprematurely and form the undesirable deposits.

A reinforcement or increase in the radial inward flow 14 is provided bythis invention. As previously indicated, the flow is upwardly from thetube sheet 8, the flow 14 therefore having both a radial force componentand an upward axial force component.

In the example of FIG. 1, such reinforcing means is provided, the tubepitch, or radial interspacing, of the tube legs 3 and 4 throughout theinnermost portion' of the tube nest, this annular area being indicatedat 16, being substantially twice as large as in the area outwardly andmore adjacent to the periphery of the tube sheet 8 and the tube nest,indicated at 17.

With this construction, the feed water entering over the upper surfaceof the tube sheet 8 and into the outer portion of the tube nest, has itsnormal upward axial component reduced because of the increase density ofthe tube pitch, or in other words, the closer interspacing of thevertical tube legs adjacent to the peripheral portion of the tube nest.The inward radial component is therefore increased or reinforcedrelative to the upward component, preventing the formation of stagnantor dead water zones adjacent to the tube sheet center l and upwardtherefrom, such as would result in pre mature conversion of water tosteam, or boiling, in those areas.

In the second example, shown by FIG. 2, four annular flat plates orbaffles are vertically interspaced, these baffle plates having outerperipheries of substantially the same diameter positioned to receive theinitial feed water flow, again indicated by arrow 14. This series ofannular baffle plates may have outer peripheries which become larger,the outer peripheries of this series being located adjacent to the lowerend 13 of the shell which, with the housing or casing 9 and the tubesheet 8, define an annular feed water inlet. These baffles are markedthrough 23, their outer peripheries having diameters approximately thesame as the diameter of the shell 10. The inside diameters of the seriesof baffles 20 through 23 progressively decrease downwardly with respectto the series of baffle plates. The lowermost baffle 20 closest to thetube sheet 8 has the smallest inside diameter, and the baffle 23, whichis farthest away, has the largest inside diameter.

With this FIG. 2 construction, the feed water flow is prevented fromupward flow throughout the outermost peripheral tube sheet and tubenest. This radial flow is gradually released inwardly, for the upwardflow. The vertical interspacing of these annular baffles is shown as allbeing the same, but this interspacing may be varied as required toassure that an adequate flow velocity exists reliably up to the centerportion 15 of the tube sheet 8 and the corridor or passage formed by thetubes above this area. With this arrangement of FIG. 2, the prior artuniform interspacing of the tube legs 3 and 4 may be maintained becausethe annular baffle plates perform the function of reinforcing the radialinward component of the flow 14 as compared to the upward or axial flow11a.

FIG. 3 shows a third example of a means for increasing the radial feedwater flow component relative to its upward axial flow component. Thisis done in this instance by making the upper surface of the tube sheet 8with a convex shape. As previously indicated, this may be done byappropriately thickening the tube sheet so that its lower surfaceremains a plane flat surface or, although not shown, by making the tubesheet with the same thickness throughout and with a correspondingconcave lower surface.

In either case, the convex upper surface increases the upward axialcomponent of the feed water flow, adjacent to the tube sheet center 15,to a greater extent than near the peripheral portion, this of necessityincreasing the radial flow component towards the center portion 15.

The convex shape may be contoured to fit the naturally developing flowas indicated in FIG. 3. That is to say, the curvature may be such thatthe convexity gradually increases in degree towards the center 15 of thetube sheet 8. In any event, the convexity should be such as toadequately increase or reinforce the radial component of the feed waterflow sufficiently to prevent localized boiling with the consequentformation of deposits which should be avoided.

What is claimed is:

1. A steam generator including an upstanding, inverted U-shaped nest oftubes through which heating fluid is passed, said tubes having verticallegs, a horizontal circular tube sheet having holes formed therethroughand in which the lower ends of said legs are mounted, an enclosure forthe tube sheets upper surface and the tube nest for containing water andgenerated steam, means for flowing feed water into the enclosureradially inwardly from the periphery of the tube sheet and over thelatters upper surface for conversion to steam by upward flow therefromthrough the tube nest, and means for flowing the heating fluid throughthe tube nests tubes via the tube sheets lower surface;

wherein the improvement comprises means for reinv forcing the inwardradial flow force of the feed water to an extent preventing itsconversion to steam locally adjacent to the central portion of the tubesheet and nest of tubes, said reinforcing means comprising a series ofvertically interspaced annular plates having outer peripheral edges ofsubstantially the same diameter all of which are positioned to receivethe initial feed water flow via all of said edges, and having innerdiameters which progressively increase from the uppermost one of saidseries downwardly to the lowermost one of the plates.

1. A steam generator including an upstanding, inverted U-shaped nest oftubes through which heating fluid is passed, said tubes having verticallegs, a horizontal circular tube sheet having holes formed therethroughand in which the lower ends of said legs are mounted, an enclosure forthe tube sheet''s upper surface and the tube nest for containing waterand generated steam, means for flowing feed water into the enclosureradially inwardly from the periphery of the tube sheet and over thelatter''s upper surface for conversion to steam by upward flow therefromthrough the tube nest, and means for flowing the heating fluid throughthe tube nests'' tubes via the tube sheet''s lower surface; wherein theimprovement comprises means for reinforcing the inward radial flow forceof the feed water to an extent preventing its conversion to steamlocally adjacent to the central portion of the tube sheet and nest oftubes, said reinforcing means comprising a series of verticallyinterspaced annular plates having outer peripheral edges ofsubstantially the same diameter all of which are positioned to receivethe initial feed water flow via all of said edges, and having innerdiameters which progressively increase from the uppermost one of saidseries downwardly to the lowermost one of the plates.